System: Allow overriding BIOS in boot parameters

src/core/system.h

@@ -43,6 +43,7 @@ struct SystemBootParameters
   std::string filename;
   std::string save_state;
   std::string override_exe;
+  std::string override_bios;
   std::optional<bool> override_fast_boot;
   std::optional<bool> override_fullscreen;
   std::optional<bool> override_start_paused;